The present invention relates generally to retroreflective signs. The invention has particular application to signs that utilize cube corner retroreflective sheeting operating on principles of total internal reflection (TIR).
The term xe2x80x9csignxe2x80x9d as used herein refers to a stand-alone article that conveys information, usually by means of alphanumeric characters, symbols, graphics, or other indicia, and that in use is mounted to an object such as a post, bracket, wall, or similar body. Specific examples include signs used for traffic control purposes (STOP, YIELD, speed limit, informational, roadside markers, etc.), street signs, and vehicle license plates. The term xe2x80x9cretroreflectivexe2x80x9d as used herein refers to the attribute of reflecting an obliquely incident light ray in a direction antiparallel to its incident direction, or nearly so, such that it returns to the light source or the immediate vicinity thereof.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes called xe2x80x9cbeadedxe2x80x9d sheeting, employs a multitude of microspheres typically at least partially imbedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes, vapor coats) to retroreflect incident light. Illustrative examples are disclosed in U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No. 5,066,098 (Kult). In contrast, cube corner retroreflective sheeting comprises a body portion typically having a substantially planar front surface and a rear structured surface comprising a plurality of cube corner elements. Each cube corner element comprises three approximately mutually perpendicular optical faces that cooperate to retroreflect incident light. Examples include U.S. Pat. No. 1,591,572 (Stimson), U.S. Pat. No. 4,588,258 (Hoopman), U.S. Pat. No. 4,775,219 (Appledorn et al.), U.S. Pat. No. 5,138,488 (Szczech), U.S. Pat. No. 5,213,872 (pricone et al.) U.S. Pat. No. 5,691,846 (Benson, Jr. et al.), and U.S. Pat. No. 5,696,627 (Benson et al.).
Some types of cube corner retroreflective sheeting require the rear structured surface to be maintained in a benign air (or other low refractive index medium) environment. This is particularly true for sheeting that relies upon TIR at the faces of the cube corner elements. Presently, this type of sheeting is generally provided with a seal film as shown in the enlarged fragmentary view of FIG. 1. Prior art sheeting 10 shown in that figure has a body portion 12 with a front surface 12a and a rear structured surface 12b, a seal film 14, a pressure sensitive adhesive (PSA) layer 16, and a release liner 18. Sheeting 10 also has a top film or layer 20 which can comprise: an ultraviolet (UV) absorbing material; a patterned ink layer, or other patterned layer such as ElectroCut(trademark) film sold by 3M Company, that forms indicia such as alphanumeric characters, symbols, or graphics; or a combination of UV absorbing and patterned or colored layers. Seal film 14 is bonded to structured surface 12b in a repeating pattern of closed polygons which form isolated, sealed cells to keep contaminants away from groups of individual cube corners on the structured surface. Boundaries of the polygonal cells are depicted at 14a. Heat and pressure used to form the cells destroys or deforms cube corner elements located along the boundaries 14a. 
In order to make a retroreflective sign using the prior art sheeting 10, a sign board is provided which comprises a rigid mass of metal, wood, plastic, or the like. xe2x80x9cSign boardxe2x80x9d as used herein means a rigid substrate suitable for mounting in the intended end use application. The retroreflective sheeting 10 is then prepared by removing the release liner 18 so as to expose the PSA layer 16. Then the sheeting is applied to a smooth, flat front surface of the sign board with the adhesive layer contacting such front surface. Letters, symbols, or other indicia may be added in layers above the body portion 12 either before or after the sheeting 10 is applied to the sign board. Some signs, such as certain road shoulder markers, carry no indicia at all but are merely bolted to a post at the side of the road.
FIG. 2 shows a fragmentary view of another prior art sheeting 22 applied to a substrate 24 such as a sign board. A body layer 26 of sheeting 22 has a rear structured surface which includes both cube corner elements 28 and a plurality of raised sections 30 arranged to define cells enclosing groups of cube corner elements. A seal film 32, ultrasonically bonded to raised sections 30, seals off these cells to maintain an air interface at the faces of the cube corner elements 28. An adhesive layer 34 bonds seal film 32, and thus sheeting 22, to substrate 24. FIG. 3 shows a fragmentary plan view of the structured surface of FIG. 2. The structured surface includes cube corner elements 28 and intersecting raised sections 30, two of which are shown in FIG. 3.
There is a continuing need to reduce the cost of retroreflective signs and to simplify the manufacture thereof.
As disclosed herein, a retroreflective sign is provided that comprises a rigid sign board and a retroreflective sheeting having a structured surface. The structured surface is bonded directly to the sign board to define a plurality of cells. The cells protect the structured surface from moisture and dirt, which could adversely impact the retroreflective performance of the film. By sealing the structured surface directly to the sign board, rather than to an intermediate seal film and then to the sign board, a simpler, prefabricated sign construction is achieved.
Preferably, the structured surface is bonded to the sign board in a repeating pattern. Raised sections defining the repeating pattern can be employed in the structured surface of the retroreflecting sheeting, or on the surface of the sign board. The raised sections help to maintain a space between at least some cube corner retroreflective elements on the structured surface and the sign board surface. Bonding can alternately be accomplished without raised sections by hot pressing the structured surface against the sign board in the repeating pattern which destroys cube corner elements along the pattern boundary. Various bonding techniques are contemplated, including ultrasonic bonding, heat sealing, and bonding via conventional adhesives.